Compositions for generating oxygen are required in restricted environments such as submarines or space capsules, or in emergency situations such as in case of sudden pressure drops in aircrafts.
In principle, oxygen generating compositions are known since a long time. Typically, the compositions produce oxygen by thermal decomposition of alkali metal chlorates or perchlorates or alkaline earth metal chlorates or perchlorates. Accessory combustible materials such as carbon or metal powders (serving as fuels, i.e. reducing agents) may be added in order to assist in obtaining the high temperatures required for the decomposition of the chlorates and/or perchlorates. Since the chlorates and/or perchlorates (halogenates (+V, +VII)), in the form of candles, slowly “burn” in the presence of reducing agents, and liberate their oxygen, the compositions are also called “oxygen candles” or “chlorate candles”.
Further usual constituents of solid oxygen generating compositions are binders or stabilizers, such as silicates and mica, for influencing the decomposition rate of the oxygen sources and assisting in maintaining structural integrity, additives for suppressing undesired side reactions or for capturing undesired side products, and catalysts such as oxides of various transition metals for speeding up reaction rates or for lowering reaction temperatures, respectively.
The compositions are molten and cast into a desired shape or, usually, press-moulded into powder compacts such as cylindrical rods, tablets, briquettes or other shapes. The shaped composition is placed into a container, and combined with a primer to yield an oxygen generator, which ideally can be easily handled and stored for a quite long time without suffering deterioration.
Once the primer (ignitor) is activated, for example mechanically or electrically, the released energy starts the halogenate decomposition reaction which proceeds without supply of auxiliary power.
Desirably, the oxygen generators shall produce an oxygen stream reliably and continuously over an extended period of time.
Further desirably, the oxygen stream shall be breathable by humans, i.e. essentially free from toxic or otherwise noxious components such as carbon monoxide, carbon dioxide, and chlorine, and shall be provided at a temperature as close to room temperature as possible.
Prior to and during the reaction, the oxygen generating composition should also be in a mechanically stable form in order to avoid any failure during activation (ignition) of the composition, and to avoid interruptions of the oxygen flow, which might result from mechanical alterations of the composition structure induced by external influences, or by modifications of the course of reaction.
A particular problem arises from the fact that in conventional oxygen generators the reaction zone in the burning candle is liquid, due to the high decomposition temperature. Irrespective of the geometrical shape of an oxygen candle, there is always a reaction zone travelling through the candle, starting at the point of ignition. The oxygen generating composition, of course, does not necessarily have the form of a candle, and in the following, the term “candle” means any mould, i.e. shaped part, for example a briquette or tablet.
Depending on the ingredients of a particular oxygen candle, which influence decomposition temperature and melting temperature of the candle, the candle may melt not only in the reaction zone, but also at some distance from the reaction zone. Thus, there is a liquid zone travelling through the candle upon decomposition.
It can be easily understood, that such liquid zone within the otherwise solid candle considerably destabilizes the candle. Mechanical shocks and even slight vibrations tend to separate the unburned portion of a candle from the burned portion, thus interrupting heat transfer, and stopping the decomposition and generation of oxygen.
Various attempts have been made to impart greater structural integrity to the burning candles and their reaction residues in order to render them less sensitive to mechanical influences, and to render them suitable also for operation under zero gravity conditions.
The method of choice used in the prior art is the addition of binders, such as asbestos fibers, infusorial earth, silicious fillers, or fiberglass. The binders also help to avoid extreme volume changes of the candles due to oxygen loss, which is particularly important if an oxygen generator does not contain one single candle, but a plurality of candles in contact with each other. In such an arrangement one burning candle ignites the next candle. Shrinkage of the burnt candle at least deteriorates contact and makes proper decomposition/combustion propagation impossible.
Up to now, however, the problem of destabilization due to localized melting during decomposition, as well as the problem of destabilization as a result of oxygen loss, has not been solved in a satisfactory manner. A further still unsolved problem is the high temperature of the oxygen evolved.